You need to change your nvarchar data into datetime by using below line of code. It's only example how can you change your nvarchar to datetime.
DECLARE @strFirstDate varchar(20),
@strSecondDate varchar(20)
SELECT @strFirstDate = '20060120112128',
@strSecondDate = '20060120112659'
SELECT CONVERT(datetime, LEFT(@strFirstDate, 8) + ' ' + substring(@strFirstDate, 9, 2) + ':' + substring(@strFirstDate, 11, 2) + ':' + substring(@strFirstDate, 13, 2)),
CONVERT(datetime, LEFT(@strSecondDate, 8) + ' ' + substring(@strSecondDate, 9, 2) + ':' + substring(@strSecondDate, 11, 2) + ':' + substring(@strSecondDate, 13, 2)),
DATEDIFF(Minute,
CONVERT(datetime, LEFT(@strFirstDate, 8) + ' ' + substring(@strFirstDate, 9, 2) + ':' + substring(@strFirstDate, 11, 2) + ':' + substring(@strFirstDate, 13, 2)),
CONVERT(datetime, LEFT(@strSecondDate, 8) + ' ' + substring(@strSecondDate, 9, 2) + ':' + substring(@strSecondDate, 11, 2) + ':' + substring(@strSecondDate, 13, 2))
)
I hope it works for you.